The present invention relates to a plurality of storage systems, in each of which a pair of storage volumes are formed, thereby multiplexing data. More particularly, the present invention relates to a technique for accepting access requests from both of a main host computer and a sub-host or backup computer while matching data redundancy is kept between those storage volumes.
In each of the storage systems developed in recent years, many of storage control units and/or storage volumes connected to such storage control units are mirrored so that such processings as online jobs, etc. that have been executed in those systems are restored quickly from errors and/or disasters that might occur in them. If such a mirrored configuration is taken for a part or the whole of a storage system, it will also be effective for maintenance works of the system.
One of the methods for forming each storage volume in such a dual configuration is to connect each of two storage control units connected to two host computers to a main volume or subvolume. When the main host computer writes data in the main volume, the storage control unit copies the data from the main volume to the subvolume. The state between the main volume and the subvolume after such a copy operation is referred to a mirrored state. The backup computer cannot access any subvolume in such a mirrored state. Such a method is disclosed in JP-A No.273242/2001
In the prior art, it has been impossible for a sub-host or backup computer to access to any mirrored storage volume until the mirrored state is reset. In other words, the sub-host or backup computer cannot access any storage volume while the storage volume is in a mirrored state.